BAGHDAD — Two Marines and a sailor were killed during combat operations in western Al-Anbar province Thursday, the U.S. military announced Friday, bringing to 18 the number of American casualties in Iraq during April.

April was the deadliest month in Iraq since September both for Iraqi civilians and U.S. soldiers, according to figures released Friday, but the overall number of attacks remains low compared with previous years of the war, the U.S. military said.

At a briefing for journalists at the U.S. Embassy in Baghdad, chief military spokesman Brig. Gen. David Perkins said the U.S. military had detected no overall increase in the number of attacks. Rather, he said, the insurgency had shifted its tactics to focus on killing large numbers of civilians.

“They’re not anywhere near the number we had a year or a year and a half ago,” he said of the attacks. “They’re going after soft targets that give them a lot of visibility.”

The Iraqi Ministry of Health reported that 355 Iraqis died in violence nationwide last month, 290 of them in bombings that included several high-profile attacks in which dozens of people died.

That’s a 41 percent increase over the number who died in March.

In the worst single attack in April, two suicide bombers detonated explosive vests among pilgrims at a Shiite Muslim shrine in northeastern Baghdad, killing 71 people. An additional 48 people died Tuesday in five car bombings across Baghdad.

Nonetheless, U.S. military casualties are sharply lower than they were during most of last year. In April 2008, 52 American service members were killed, according to the Web site icasualties.org, which monitors U.S. and coalition death tolls in Iraq and Afghanistan.

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